Flow rate measurement is important in many fields. For example, many industrial processes require measurement of flow rate through various conduits in order to control the process appropriately. Other uses requiring measurement of a liquid or gas include delivery of a product to a consumer, such as gas, oil, and water.
In the medical field, liquid measurement is sometimes applied to a patient's urine output or to a medicine being administered intravenously. Acute kidney injury (AKI) is a common problem in hospitalized patients, particularly in critical care and in the operating room. However, only recently has the medical profession formulated criteria for assessing and classifying the risk and progression of AKI. These criteria specify five major stages in the progression of AKI: Risk, Injury, Failure, Loss, End stage renal disease, (known by their initials as the RIFLE criteria). Based on the success of RIFLE, the Acute Kidney Injury Network—AKIN—was formed by an international cadre of leading clinicians focused on the issue of AKI. AKIN has endorsed and promoted RIFLE. In addition, they also proposed minor modifications to the RIFLE criteria which they refer to as the AKIN criteria. The RIFLE-AKIN criteria provide valuable tools for preventing AKI. These criteria include measurements of creatinine clearance and urine output. Creatinine clearance is a very late indicator showing only that AKI has already occurred. Urine output as a measurement of kidney function is typically assessed on a daily or shift-wise (e.g., eight-hours) basis.
Thermal transfer flow rate meters typically measure flow rate continuously using a heating element and two temperature sensors (one upstream and one downstream from, or adjacent to, the heater). By measuring the temperature differential between the two thermometers, the flow rate is calculated. Alternatively, the temperature is kept at a constant value above the ambient temperature of the fluid at the heater and the energy required to do so is monitored, from which the flow rate can be calculated.
FIG. 1 schematically shows the basic arrangement of a prior art thermal mass flow rate meter. A liquid flows through a tube 100 in a direction indicated by the arrows. At some location in a wall of the tube or in the interior of the tube is placed a heating element 120. Temperature sensor 110, which measures temperature Ti, and temperature sensor 112, which measures temperature Tj, are located respectively upstream and downstream of heater 120. Isothermal lines 130, 131, and 132 symbolically show the temperature distribution as a result of the power input to the heating element, where the T130>T131>T132.
The calculation for determining the flow rate is according to the equation:Q=Cp·m·ΔT  equation 1substituting m=ρVQ=Cp·ρV·ΔT  equation 2dividing both sides by t and solving for {dot over (V)}=V/t{dot over (V)}=Q÷[t·ρ·Cp·(Tj−Ti)]  equation 3noting that Q =I·v·t{dot over (V)}=I·v÷[ρ·Cp·(Tj−Ti)]  equation 4wherein the symbols used herein are defined in the following table:
SymbolMeaningUnitsVVolume[l] Liters {dot over (v)}Volumetric Flow Rate (volume/time)      [          l      min        ]    ⁢          ⁢  Liters  ⁢      /    ⁢  minute QEnergy, work[J] Joules ρDensity      [          g      l        ]    ⁢          ⁢  grams  ⁢      /    ⁢  liter CpSpecific Heat Capacity (under constant pressure)      [          J                        g          ·          °                ⁢                                  ⁢                  C          .                      ]    ⁢          ⁢  Joules  ⁢      /    ⁢      (                  gram        ·        °            ⁢                          ⁢              C        .              )   TTemperature[° C.] degrees CelsiusTiTemperature of liquid before[° C.] degrees Celsiusthe heater (upstream)TjTemperature of liquid after or[° C.] degrees Celsiusat the heater (downstream)IElectric Current[A] AmperesvElectric potential[v] VoltsΔTTemperature Difference Tj−Ti[° C.] degrees CelsiustTime[s] seconds
A related type of thermal transfer flow rate meter, known, inter alia, as a constant temperature flow rate meter, uses a similar arrangement to that shown in FIG. 1 with the exception that temperature sensor 112 is adjacent to, or integral with heating element 120. In this configuration, the heating element 120 is heated to a set constant differential temperature Tj (as measured by sensor 112) above the temperature Ti measured by sensor 110. The amount of heat carried away by the flowing fluid depends on the flow rate. The temperature of heater 120 is kept constant by adjusting the current applied thereto. The value of the electric current (I) required in order to maintain a constant temperature differential ΔT provides a means to calculate the flow rate, as shown in equation 4.
In the above basic arrangement of a prior art thermal mass flow rate meter a quantity of heat is applied to the fluid by heating element 120 until the temperature reaches a value Tj. At this point the heating element is turned off and the time is measured until the temperature returns to the original value Ti. The time of the first measuring point is accurately known but it is difficult to determine the exact time at which the second measurement should be taken, since the temperature changes relatively slowly as it approaches its steady state value. Furthermore, when making repeated measurements of the flow rate the ambient temperature of the liquid may slowly increase, thus an accurate value of Ti of the liquid is not obtained. Moreover, the use of a prior art thermal mass flow rate meter, which requires that energy be continuously applied to the heating element, is not energy efficient.
It is therefore an object of the present invention to provide a simple and accurate method for determining the flow rate of a liquid.
It is another object of the invention to provide a simple, cost effective and accurate flow rate meter.
It is another object of the invention to provide a flow rate meter that has a minimal energy requirement.
Further purposes and advantages of this invention will appear as the description proceeds.